1. Field of the Invention
This invention relates to the art of a magnetic north detection for detecting the magnetic north direction, particularly, relates to a magnetic north detecting device and a magnetic north detecting method for detecting a magnetic north direction, based on geomagnetism magnetic filed intensity measured by a 3-dimensional geomagnetism sensor. This invention is applied to mainly to cellular phones.
2. Description of the Related Art
Cellular phones having navigation function are put into practical use. The navigation function usesposition information obtained by a predetermined position detecting means and magnetic north direction calculated by a magnetic north direction sensor for detecting the magnetic north direction based on measuring data of the geomagnetism. As shown in FIG. 1, the magnetic north direction sensor comprises a 2-dimensional geomagnetism sensor unit 1 for detecting geomagnetism magnetic field intensities of two directions perpendicular to each other and a 2-dimensional operation functional section 2 which calculates the magnetic north direction from the geomagnetism magnetic field intensities measured by two geomagnetism sensors included in the 2-dimensional geomagnetism sensor unit 1. The particular magnetic north direction sensor is disclosed in, for example, Application Note AN00022 “Electronic Compass Design using KMZ51 and KMZ52” published by Philips Semiconductors.
The calculation method of the 2-dimensional operation functional section 2 utilizes the following principle. Specifically, when an angle formed by the magnetic north direction and an axis of the sensor is denoted by, for example, Θ, one of two geomagnetism sensors placed on the 2-dimensional plane shows magnetic field intensity proportional to sin(Θ), while the other shows magnetic field intensity proportional to cos(Θ). In order to calculate the magnetic north direction angle Θ backward from the magnetic field intensities measured by two geomagnetism sensors, the ratio of the magnetic field intensities is fined, and tan-1 of the ratio is calculated.
The above-mentioned art, however, requires the premise that the 2-dimensional geomagnetism sensor unit is level to the earth surface. When the sensor unit 1 is not placed on a level to the earth surface, it produces an error in reading of the magnetic north direction. The reason of producing the error is that the geomagnetism direction is not level to the earth surface and, therefore, is inclined to the earth surface by so-called “dip angle”. For example, the dip angle in Japan and environs is distributed in a range of about 44 to 59 degrees. In the case of the dip angle of 45 degrees, horizontal and perpendicular components of the geomagnetism magnetic field are equal to each other. Therefore, the perpendicular component is substantially equal to or larger than the horizontal component in Japan and environs. For this reason, when the cellar phone is not being kept level to the earth surface and, thus, the 2-dimensional plane on which the 2-dimensional geomagnetism sensor unit is placed is inclined to the earth surface, i.e., when the geomagnetism sensor unit is inclined to the earth surface, a big error arises in indication of a magnetic north direction. Thus, the error of the magnetic north direction due to the 2-dimensional geomagnetism sensor unit not being kept level to the earth surface is one of the interfering factors in gaining the market share of cellar phones having a magnetic north detecting function.